1. Field of the Invention
The present invention relates to a digital TV receiver, and more particularly to a robust symbol timing recovery system for Vestigial Side Band (“VSB”) receivers where symbol timing is recovered regardless of carrier phase offset or jitter caused by imperfect carrier recovery
2. Description of Related Art
The ATSC: A/53 Digital Television Standard, was developed by the “Digital HDTV Alliance” of U.S. television vendors, and has been accepted as the standard for terrestrial transmission of HDTV signals in the United States. The ATSC A/53 standard is based on an 8-level vestigial sideband (8-VSB) modulation format with a nominal payload data rate of 19.4 Mbps in a 6 MHz channel. Synchronization including timing and carrier recovery are essential parts in extracting the transmitted symbols from the received signal. In the 8VSB-T transmission system, a pilot signal is added to help the receiver with the carrier recovery (CR). Usually in a VSB receiving system, symbol timing recovery (TR) is accomplished jointly with CR or follows the CR. During synchronization, the residual carrier phase offset and/or jitter often passes through the CR block and degrades the performance of the TR block (and consequently the overall receiver performance).
Unlike VSB systems, a receiver for QAM does not suffer from this phenomenon because symbol information is conveyed independently through the in-phase (I) and quadrature-phase (Q) channels. The well known TR methods such as spectral line extraction or Gardner's algorithm use both channels (I&Q phase) simultaneously canceling out the carrier phase term. However in VSB systems, symbol information is mainly contained in I channel. The Q channel, which is just a Hilbert transform of the I channel, is employed in order to reduce the transmission bandwidth. Thus, the timing information contained in I and Q channels are not independent of each other, making it impossible to cancel out the phase offset term while extracting timing error information with a conventional TR method.
For this reason, most VSB receivers use only the I channel signal for symbol timing recovery. This works well when the carrier phase offset does not exist or is negligible, but is otherwise highly problematic. Thus, a need exists for a robust timing error detection scheme for VSB receivers that can generate exact tuning errors regardless of carrier recovery status. According to the prior art, as described above, the performance of VSB receivers would undergo a degradation when carrier recovery is not accomplished perfectly.